In a typical Internet Protocol (IP) network, packets are sent with an IP header that describes a Layer 3 source (Open Systems Interconnection (OSI) reference model) and destination IP addresses of the packet as well as other information. At an IP hop, the router looks up the IP destination address in a routing table and the route obtained is used to send the packet toward the next hop on the way to the destination. IP routing is connectionless, that is, all routers participating in the routing domain distribute routing information to each other and packets intended for the same destination may follow different individual paths through the routers toward the destination.
A common IP network technique is “tunneling,” where one packet is encapsulated into another packet by adding a new packet header (and possibly trailer). This technique is useful, for example, for sending packets over networks having different networking technologies and/or when a group of packets are all to receive the same treatment by the network.
The Multi-protocol Label Switching (MPLS) technology, a method of tunneling, provides circuit-like properties to connectionless IP networks. MPLS is commonly used as a tunneling technology, that is, MPLS tunnels are used in a wide variety of circumstances. MPLS technology encapsulates packets and attaches a label stack where the top-most label in the label stack determines the next routing decision rather than indicating an IP destination address, though it is not necessary that the data packets be IP packets. A tunneled packet, that is, the packet inside the MPLS encapsulation is not necessarily an IP packet In general, the signaling protocols used with MPLS are Reservation Protocol/Traffic Engineering (RSVP/TE), which is a variation of the Reservation Protocol (RSVP) and Label Distribution Protocol (LDP). Added services may also be signaled, such as alternate tunnel paths.
Routers in the core network examine the label stack and forward the packet according to the label's direction. The forwarding router does not look up the destination or hext hop in an address table. A destination router receives the data packet and removes the label before sending the packet on to the data's intended receiver. Traffic engineering (TE) is concerned with optimizing the use of the existing capacity of the network. For example, overloaded links are relieved of traffic pressure while underused links carry more of the traffic load. TE is also used to direct traffic with specific Quality of Service (QoS) needs toward network resources, i.e., nodes and links, which support those needs. If needed, network resources are reserved to help guarantee the QoS requested. RSVP/TE and MPLS tunnels are commonly applied in TE solutions.
Modern IP Traffic Engineering solutions incorporate Differentiated Services (DS) and combine these with MPLS (DSMPLS). This combination provides mechanisms for tracking available link bandwidths in several classes of service, which in turn, enables constraint based routing (also called constrained, shortest path first (CSPF) routing). CSPF finds paths through the network where sufficient bandwidth exists in the desired class. An MPLS tunnel may then be established along these CSPF paths. Thus, TE can be applied to different classes of traffic and the MPLS tunnels support this application by carrying traffic of one or more classes of service.
Routers are sometimes classified as provider edge (PE) and customer edge (CE) depending on whether the Internet service provider or customer operates the routers. In a traditional MPLS paradigm, PE routers provide MPLS functionality and run MPLS signaling protocols while CE routers are assumed to not have MPLS functionality. This assumption is made for two reasons. The first is backward compatibility, that is, the providers wished to use more sophisticated MPLS/TE solutions in their own networks without requiring their customers to change or upgrade the customer's CE routers. The second reason is to support more efficient scaling because the PE's could be richly meshed and this MPLS network topology could support many more CE routers than PE routers. This paradigm assumes that MPLS is used in the core but that it is not needed in the customer access networks.
Pseudo Wire Emulation Edge to Edge (PWE3) technology provides Layer 1 and Layer 2 specific emulation services over packet switched network (PSN) tunnels. Associated with PWE3 is the pseudo-wire control protocol (PWE3CTL), which is based on Label Distribution Protocol LDP. PWE3 allows a provider to offer different Layer 1 and Layer 2 services, i.e., an E1 circuit, an ATM VCC, a 100 Mb Ethernet and a Frame Relay PVC may all be offered over a common IP/MPLS core infrastructure. These are referred to as a TDM pseudo-wire (PW), an ATM VCC pseudo-wire, an Ethernet pseudo-wire and a Frame Relay pseudo-wire respectively. This is accomplished by the PWE3 pre-pending a pseudo-wire header, which is the last label on the MPLS label stack. The PW header identifies the pseudo-wire over which the encapsulated data is to be transported, which in conjunction with pseudo-wire signaling allows the remote end points of the PW to understand how the particular Layer 1 or Layer 2 encapsulation details are to be handled.
PWE3 relies on tunneling technology to transport the PWs across the MPLS/IP core. MPLS is a popular choice for this transport because of the possibility of using Diffserv aware MPLS TE tunnels, so that PWs with different characteristics can be aggregated into MPLS tunnels that support an appropriate class of service. When PWE3 uses MPLS tunnels, it also assumes the MPLS paradigm that MPLS is used in the core but is not needed in the customer access networks.
As stated above, a PE router is typically the device that offers the Layer 1 or Layer 2 services to a like customer premises device. Typically the provider offers several Layer 1 or Layer 2 emulated services and the provider is managing the resultant complexity. The equipment cost to enhance a PE router to provide the speed and the feed for each service is considerable. Each PE router used to offer the Layer 1 or 2 services must be enhanced. Typically additional PE routers must be installed to simply obtain port density needed for an additional service offering and the routing capacity of these PE routers may not be fully utilized.
In its current usage PWE3 capability is typically concentrated in a provider edge (PE) router. These routers are used to provide Layer 2 and Layer 1 services via native attachment media. (e.g., the PE router provides ATM service using an ATM port on the PE router. The ATM traffic is then encapsulated in PWE3 and transported). PE routers generally do not have the space for the volume and types of native interfaces that would be needed for cost effective service deployment. Typically, a second PE router must be added to get additional interfaces before the forwarding capacity of the first PE router is fully utilized. What is needed is a relatively inexpensive means for providing ports and interfaces to improve routing capacity, cost efficiency and scalability. At the same time this means for providing ports and interfaces must be standards compliant and backward compatible with existing PW equipment.